I applied the pigments in 3 different ways, depending the surfaces to treat:

First, on the lower hull, I added some pigments in a random way. Once it is all set, I used a brush dipped in Tamiya thinner and fixed the pigments onto the surface by capillarity. Some areas got "pigment free". Don't worry, you just have to spread some pigments over the surface. This gives a "crunchier" look to the mud, rather than monotonous rolls, resulting from the mix of the different pigments with thinner.

Finally on the upper parts, I applied some pigments in more or less thick layers that I fixed with Tamiya Thinner that I airbrushed. A low pressure is enough, start from far and get closer and closer to the surface. This gives an interesting effect, but you'll need some practice!